This invention relates to a protection system for an electric motor, and in particular, to a system having a fault sensing device directly connected to the motor and having a second device to indirectly sense the opening of the fault sensing device upon the occurrence of a sensed fault.
Air conditioning systems, including mechanical refrigeration units, have grown in importance in maintaining the environment of enclosed areas comfortable for the occupants thereof. Very often, the mechanical refrigeration unit is physically located at a remote distance from the inhabited areas served by the air conditioning system. For example, in shopping malls, very often the refrigeration units are located on the roof of the mall structure. When installed on a roof or similar location, the refrigeration unit is relatively inaccessible and thus operates with minimal amount of human monitoring.
Typically, the mechanical refrigeration unit includes safety controls to deenergize the electric motor employed to drive the compressor upon the occurrence of a fault or detrimental operating condition. For example, if the compressor discharge pressure exceeds a predetermined magnitude, the motor is rendered inoperable. Similarly, the motor is stopped if the temperature of the motor's windings exceeds a predetermined value. Very often, the safety controls are of the type which automatically reset upon the passage of a predetermined time interval. If the fault should reoccur, the safety control will again deenergize the motor. This undesired cyclical operation may occur for a prolonged period of time without detection, whereby eventual damage to the compressor or motor may result.
To prevent the undesirable cyclical operation resulting from an undetected fault, many controls have heretofore included lock-out mechanisms whereby, once a fault occurs and the motor is deenergized, the motor is prevented from being restarted until a reset mechanism is manually actuated. By employing a manual reset device, the operator or maintainer of the air conditioning system is alerted to the potential problem. For convenience purposes it may be desirable to have the reset device located in the occupied areas, remote from the refrigeration unit. The thermostatic switch of the air conditioning system, in combination with a relay, has been employed to obtain the desired reset function. An example of such a control is illustrated in U.S. Pat. No. 3,050,254, issued Aug. 21, 1962. Although, the control disclosed in the aforecited patent illustrates the desired lock-out feature in combination with a manual reset mechanism, the illustrated control is not entirely satisfactory if employed with modern refrigeration equipment.
Typically, a large percentage of present refrigeration equipment employ hermetically sealed motor compressor units. Almost all such motor compressor units have safety devices provided to prevent the continued operation when the temperature of the motor's winding has exceeded a predetermined level. It is desirable that such safety device be mounted in direct contact with the motor's windings and thus be sealed within the same shell as the motor compressor unit. The aforecited patent does not disclose a safety device to directly sense the winding temperature and to deenergize the motor compressor unit upon excessive winding temperature. The high-low pressure cut-off switch therein disclosed is connected in the circuit at a point remote from the motor compressor unit. Unlike the high-low pressure cut-off switch which is employed to open the electrical circuit to the motor and to energize a lock-out device, a safety device to deenergize a hermetically sealed motor upon excessive winding temperature may not feasibly be employed to both deenergize the motor, to actuate a lock-out feature. Thus, even if the cut-off switch disclosed in U.S. Pat. No. 3,050,254 were mounted in direct contact with the windings, the control therein disclosed cannot be readily employed to obtain the desired protection, lock-out, and reset functions.